How to Select a Custom Magnet Sensor

The success of any system that utilizes magnetic sensors is dependent upon the ability of the design engineer to completely grasp the unique parameters, requirements, and constraints of the entire system, not just the sensor. Beyond answering the simple, yet essential question of “What does this sensor have to do?”, it is important to define the environment in which the sensor will be asked to function. The variables of temperature, humidity, exposure to UV, and proximity to other system components, amongst others, all must be defined so that proper sensor selection can be guaranteed.

However, if sensor selection is more of an afterthought of the OEM during the design process, it is likely that a specialized sensor will either have to be developed or modified to meet the specific requirements of the actuator system design, which can be costly. Most engineers have been faced with the predicament of finishing a design only to find that a key component of the design was overlooked, and then have to go back to the drawing board or CAD station to figure out a way to make it all fit. It is much more sensible and less costly to account for all of the potential factors that will affect the performance of a sensor prior to designing an actuator system, while always keeping a focus on the entire system design. In the cases where the use of a custom sensor can't be avoided, there are certain design parameters that should be considered to ensure a robust system design: environmental, mechanical, electrical and magnetic.

Figure 1: Components of a Hall effect sensor for a speed sensing application. As the magnet rotates over the sensor, the Hall chip detects the magnetic field.

Figure 2: A Reed switch sensor consists of contacts on metal reeds that are sealed in a glass capsule. When in proximity to a magnetic field, the contacts open or close.

Environmental Considerations: Know Your Surroundings

There are several environmental factors that could impact the performance of the magnetic sensor. Understanding the effect that any one of these factors may have on a sensor is crucial in the selection process and will help the engineer take precautions to ensure the integrity of the circuit design.

Temperature: Operating and storage temperatures that exceed 100°C could degrade the performance of the sensor and magnet. Hall effect sensors have maximum temperature limitations ranging from 85°C to 150°C. Once the maximum temperature value is exceeded, the sensor loses its calibration and will not respond properly to the magnetic field. Reed switch performance is minimally affected by temperatures up to 150°C. The final section on magnetic parameters provides more details on how temperatures impact the selection of the actuator’s magnetic material.

Humidity: This greatly influences the selection of the magnetic material and the potential coating, if required, for the magnet. Some magnet materials such as neodymium are very sensitive to moisture and can disintegrate because the iron degrades with moisture.

Shock and vibration: These dynamic forces need to be taken into consideration. Reed switches can be negatively affected by high G forces and could require special orientation of the blades.

UV: For outdoor mounting, the plastic material under consideration for the sensor should be able to withstand prolonged UV exposure. The engineer should check to see if there are any special validation performance criteria required.

Thermal shock: A thermal shock test is recommended to validate the sensor design and ensure its long-term performance. This test is usually conducted if there is a wide temperature range such as -40°C to +105°C. The high temperature may even reach +150°C. If packaged improperly, the materials can degrade under high stress. The thermal expansion coefficients for plastics and potting materials should be analyzed to ensure their compatibility with the switching component within the sensor across the full temperature range.

Mechanical Concerns: Understanding the Spatial Requirements of the Sensor

The design engineer should review the 3D CAD model showing the area where the sensor and magnetic actuator will be located within the OEM system, so that a clear understanding of how the sensor will need to operate in its given space can be developed. This process will help the engineer select the optimal sensor and magnetic actuator design for the system. To understand the mechanical constraints of the application, design engineers should ask themselves these questions:

Is there a specific orientation of the sensing device that may influence the package design and mounting method?

Is there space available for both the sensor and the actuator? Are there dimensional constraints involved?

Does the required activation and deactivation distance between the sensor and actuator allow for the proper orientation of the magnet to the sensor?

Does the customer require a special connector (i.e., a sealed type or termination) that is compatible with the mating harness or circuit board? (The wire insulation type is important when considering the temperature or chemical environments that the sensor may be subjected to. A surface-mount device could be considered for circuit-board mounting.)

Are there special materials that should be considered for meeting the mechanical requirements? Thermoplastics and nonferrous grades of metals are common materials for housing magnetic sensors such as reed or Hall effect sensors.

In addition, potting materials are needed to protect reed and Hall effect switches. There are many grades of epoxies or urethanes available for potting materials. Epoxies are usually the preferred choice for sensor protection because of their stability over a wide range of temperatures.

Is the circuit powered by a battery? If so, what is the battery voltage value?

Is the switching load resistive, inductive or capacitive?

How many switching cycles over the lifetime of the product are required?

Are there any special switching speed requirements (i.e., for speed sensors)?

Which type of sensor output does the customer desire: digital or analog?

Does the customer want a normally open, normally closed or single-pull-double-throw reed switch?

Are there any special requirements such as EMC, EMI or ESD considerations within the application? (This is very important when using a Hall effect sensor.)

Magnetic Parameters: Choosing the Best Magnet

Unfortunately, the magnetic actuator is often overlooked when specifying a reed switch or Hall effect sensor. This is especially surprising since the magnet and sensor each make up 50 percent of the final design of the magnetic circuit. Spending considerable time diligently selecting a proper sensor can all be for naught if it is paired with an improper magnet.

The OEM customer may say that he or she already has a magnet designed into the application without considering the possibility of technical issues. To prevent issues, it is important for the design engineer to review the magnet’s design with the customer for the required application. This thorough analysis of the magnetic circuit will prevent long-term quality issues during production. The engineer should use this list to assess the quality and performance of the magnet:

Use magnetic simulation software to gain a thorough understanding of a sensor’s magnetic operating environment under a wide range of conditions. This tool characterizes sensor performance while operating and interacting with magnetic fields in motion. It should consider any ferrous metals located near the sensor and magnetic actuator assembly that could shunt the intended magnetic field away from the sensor. This simulation should be done prior to investing significant time in benchtop testing and tooling.

Consider the temperature of the application since it has the most impact on the magnetic material. Some magnetic materials such as neodymium have limitations on the maximum allowable operating temperature. Once the maximum temperature has been exceeded, the flux density significantly decreases and cannot be reversed. For benign environments such as indoor applications, an alnico magnet or a low-cost ceramic magnet may work well if the material meets the required distance tolerance for sensor activation. In a high-temperature application with many fluctuations in temperature (i.e., an automotive environment), a very stable rare earth magnetic material such as samarium cobalt is often required.

Study the magnet’s polarity and orientation. A Hall effect sensor typically operates using only the north or south pole of the magnet. A reed sensor is not polarity sensitive and will operate with any magnetic pole.

If an activation tolerance is quite tight, eliminate the ceramic grades and alnico grades, while recommending a neodymium or samarium-cobalt magnet. The rare earth materials are much more cost competitive than in previous years.

Be very careful of using a neo-grade magnet when the application is within a high-moisture environment. Nickel plating over neodymium is a good barrier. However, it is not a guarantee that moisture will not penetrate under the nickel through a crack. Select a samarium-grade material for this environment if a rare earth material is required.

When packaging the magnet in a sealed plastic or nonferrous metal package, neodymium may be considered for a high-moisture environment.

To prevent price from becoming a future issue, make sure the magnet material is not overspecified for the application.

Conclusion

The key to engineering a reliable and robust magnetic sensing system is the ability to grasp all of the operating parameters required of a particular sensor in the role in which it is being asked to function. It is critical that communication between the OEM supplier and the design engineer is kept open so that proper sensor and magnet selection is ensured. In those cases where custom sensors are required, the environmental, mechanical, electrical, and magnetic specifications need to be incorporated into the sensor selection process to ensure optimal system design.